Create a component/view for displaying all active subscriptions. See #304 (comment).

Note this work should wait until:

• The workflow component has settled.
• The subscription system has settled.
• The subscription system has more advanced way of targeting workflows?

The result could be a tree something along the lines of:

• workspace1
  • tree#1234
    • a { b c }
  • dot#2345
    • a { b d }
    • a { b e }
• workspace2
  • ...
• Combined query
  • a { b c d e }

In the above workspace refers to the "page" e.g. dashboard, workflow1, etc.

As part of this work we might want to consider committing subscriptions to the central data store rather than storing them in the state of the workflow service?

See also #304 (comment)

Pull requests welcome!